Digital trunked radio communication systems are commonly used by public service organizations, such as police, fire, and ambulance squads, and by many private organizations to communicate with each other. Digital trunked radio systems provide an efficient means to communicate between single users and/or groups of users. They allow for one call to be made to many users simultaneously, such as a police dispatcher sending out a call to a group of officers at once. Any officer receiving the call has the ability to respond to the dispatcher, as well as to all other officers in the group. This makes this type of communication system well suited for public safety and municipal applications.
Digital trunked radio systems operate by allowing a user to transfer a voice call (or data call) to another user or group of users on the system. The information is transferred on one of a plurality of channels, referred to as working channels. A control channel assigns a working channel to every transmission, and notifies both the transmitting radio and all receiving radios of the working channel assignment. When the transmission is completed, the assigned working channel is released, and thus becomes available for a new transmission.
Each radio on the system has a particular logical identifier (LID) that identifies it within the system. The LID identifies each radio to the system such that the system is aware of the origin of any transmission, similar to a caller ID function commonly used in the telephone industry. It also allows for a transmission to be sent to one particular radio. Further, it allows for a radio that has been lost or stolen to be removed from the system, so that an unauthorized person in possession of the lost radio cannot listen in on or interfere with the rest of the system.
In addition to having an individual LID, the various radios can be grouped together for more efficient communications by assigning group identifiers (GID). FIG. 1 illustrates a digital trunked radio system operating in a typical municipal environment containing several groups. For example, a municipality might use a digitally trunked radio system for many departments, including both the police department and fire department. Using a GID, the individual radios can be grouped into separate groups for dividing the radios used by the police officers and the radios used by the firemen. A dispatcher can send a message to all radios identified to be in the police officer group. A message is sent out over the control channel (which is monitored by all radios on the system) identifying the GID for the radios that the transmission is intended to reach (i.e., all police officers). Those radios change to the assigned working channel to receive the message, while all other radios ignore the communication on the control channel.
In addition, the groups can be further broken down into subgroups, if desired. For example, the police officers group might consist of patrollers and detectives. Using additional GIDs, the group of police officer radios can be divided into patroller radios and detective radios. This makes it possible for a dispatcher to send a transmission to only patrollers, or only detectives.
Currently, on some digital trunked radio systems, the LID and GID assignments are achieved using a relatively short binary address. For example, the Enhanced Digital Access Communication System (EDACS) by M/A-COM Private Radio Systems, Inc. (Lynchburg, Va.) uses a 14-bit binary code to provide each radio with an LID. This creates 214 possible distinct LIDs, which allows for 16,384 radios to reside on the digital trunked system. EDACS also uses an 11-bit binary code to create various GID assignments, allowing for 211 or 2,048 possible groups to exist within the system.
When digital trunked communication systems such as EDACS were first developed, very few users required more than 16,384 radios or 2,048 groups within a system. By keeping the LIDs and GIDs to a minimum number of bits, it was possible to transmit the information required to request a channel in short transmissions (30 millisec in EDACS) which allowed a large increase in system loading capacity over conventional radio systems and other competitive trunking systems. However, as these systems have been expanded to cover larger systems and wider areas (today often an entire state or several states), the need to have additional radios and additional groups on the same system has arisen. One possibility for adding more radios to a system is to increase the user address space and group address space to include more bits by defining a new timing structure to the transmission protocols. However, doing so has some drawbacks. By defining a new structure to the transmission protocol, it may not be possible to simply upgrade old radios to operate on the new system. In addition, increasing the length of the transmission protocol could result in a slower response time or lower loading performance. Current users of systems such as EDACS might need to replace their entire system once their requirements exceed the maximum 16,384 radios or 2,048 groups. This would require replacing many thousands of radios; thus, this is not a feasible solution for most existing users. For this reason, the digital trunked radio systems remained limited in the number of radios and groups that could be present on the system as a result of the bit limitations of the LIDs and GIDs in the communication protocol.
Existing digital trunked radio systems such as EDACS use a “Slotted Aloha” protocol for communication between radios and the base station. Slotted Aloha is well known in the art, and involves transmitting information in timed groups known as “slots” or “buckets.” The standard bucket used by digital trunked radios systems such as EDACS is 30 msec in duration. This time frame equates to 288 bits of data using a standard 9600 baud rate.
A number of pieces of information are transmitted on the control channel within each bucket. Many bits are used for dotting and barker functions. Dotting allows the receiving radio to find the location of distinct bits (i.e., bit sync-ing) and barker messages allow for the receiving radio to determine the beginning of a transmitted message (i.e., word sync-ing). In addition to dotting and barker bits, information regarding channel assignment, group assignment, and individual radio identification are all transmitted within a bucket on the control channel. All of this information needs to be kept within 30 millisecond buckets in order to operate within the existing structure of the Slotted Aloha protocol used by the prior art radios currently in existence.
What is desired is a method of increasing available LID and GID addresses, while remaining within the same timing structure of the transmission protocols currently used by systems such as EDACS. In this manner, the systems could be expanded while still allowing for existing hardware to be programmed to function with the system. This would remove the need to replace existing components, while still allowing systems to be increased in size to larger than the current limitations on the number of radios and the number of groups.